Abstract

Recent surveys of Australian and South African rivers have revealed numerous Clade 6 Phytophthoras, which either have ITS gene regions that were highly polymorphic or could not be sequenced. These isolates were suspected to be hybrids. In order to establish the hybrid nature of these isolates, three nuclear loci and one mitochondrial locus were amplifiied and, in the case of the nuclear gene regions cloned, and sequenced. Abundant recombination within the ITS region was observed and this combined with phylogenetic comparison of other three loci confirmed the presence of four distinct hybrids involving three known parental species: P. amnicola, P. thermophila and P. taxon PgChlamydo. In each case the hybrid is between two parental species. For the single copy nuclear genes (ASF and GPA) examined, two alleles were obtained, one of which corresponded to each of the parental species. In all cases, only a single coxI allele was obtained indicating that mitochondria were always uniparentally inherited from one of the nuclear parents. This pattern of nuclear and mitochondrial inheritance suggests that each hybrid is a result of an independent hybridization event involving two parental species. The hybrid species are sterile and have physiological traits similar to those of the maternal parental. The pathogenicity of these hybrids is unknown, but several isolates from Western Australia were obtained from the rhizosphere soil of dying plants. The serendipitous and simultaneous discovery of the same hybrid complex on two continents is intriguing. However, the wide geographic distribution, frequent isolation and presence of all four hybrids and all parental species suggest that their origin lies in Australia. The association of the sampled riverways with botanical gardens in South Africa containing Australian plants may be a clue to the pathway of introduction.